Composite cellulose material and method for making such a material

ABSTRACT

The present invention relates to a composite cellulose material, comprising: —a layer of a cellulose material, —a starch-based reinforcement grid which is positioned on at least one surface of the layer of cellulose material, the reinforcement grid comprising a plurality of meshes which are delimited by grid wires, the composite cellulose material having a relief in three dimensions comprising folds in the positioning zones of the reinforcement grid, and relief bumps on either side of the plane of the grid which are delimited by the folds.

TECHNICAL FIELD OF THE INVENTION

The present invention concerns a composite cellulose material, various products manufactured in such a material such as a package for food, cosmetic or pharmaceutical products, paper for bags, sachets and pouches, a core for honeycomb sandwich panels or multi-ply paper towel or toilet paper structure, or wallpaper, printed paper for promotional use such as papers for posters, inserts or leaflets, papers for domestic use such as paper napkins, paper items such as book covers or an envelope, for example, as well as a method for manufacturing such a material.

STATE OF THE ART

Grid-like structures or reinforcements are widely present in nature, especially in the leaves of certain plants where they support the body of the leaves, or even in the wings of some insects. Humans were inspired by this to create plate or shell structures used in many industrial sectors, especially in transport (marine, boating, aeronautics, aerospace) and civil engineering.

These structures are made up of a skin and a network of reinforcing ribs extending over a surface of the skin, being interesting for applications that simultaneously require rigidity, solidity and lightness. This arrangement makes it possible to put material where it is necessary: when such a structure is subjected to torsion or bending, the skin essentially works in so-called membrane deformation mode, while the network of ribs essentially works in bending/torsion. It is therefore possible to obtain parts, most often in the form of flat or curved ultralight panels exhibiting very good specific mechanical properties (i.e., related to their density).

The mechanical properties of bending deformation (curving of the structure) and in torsion (twisting of the structure), as well as the buckling behavior of ribbed structures are especially determined by the geometry of the rib network. These properties are optimized through the development of geometries suited to the stresses [1]. For example, specific geometric patterns can confer an auxetic behavior [2], i.e. presenting a negative Poisson's ratio, to the structures they constitute and significantly improve some of their geometric and mechanical properties.

These ribbed structures are notably applied in the paper industry. On this topic, document FR 2 250 853 describes a method to improve the mechanical properties of a sheet of paper. This method consists of forming a reinforcement in the sheet of paper made up of a regular network of continuous thin lines of a binder commonly used in the paper industry.

The binder, especially a polyvinyl alcohol or a rubber, penetrates into the sheet of paper, and leads to the formation of a reinforcement within the sheet of paper. The reinforcement, integrated into the sheet of paper, gives this paper an increased tensile strength, while preserving the flexibility of the sheet, especially in bending and torsion.

In document FR 2 250 853, the reinforcement is formed in the sheet of paper, at the heart of said sheet of paper. The resulting reinforced paper sheet exhibits improved in-plane mechanical properties, including improved tensile strength. In return, the out-of-plane mechanical properties of the sheet, such and bending and torsional resistance, remain unchanged and therefore low since they approximately correspond to those of the initial paper. The reinforced sheet of paper is therefore not rigid, and as a result, is not suitable for use in fields where the paper undergoes a great deal of stress, in particular stress perpendicular to the plane of the sheet of paper such as bending or torsion. This is especially the case when the paper is used for the manufacture of packaging, in particular for transport or storage of food products.

BRIEF DESCRIPTION OF THE INVENTION

One goal of the invention is to propose a composite cellulose material making it possible to overcome the disadvantages described previously.

The invention especially seeks to propose such a composite cellulose material comprising a layer of cellulose material, which exhibits increased out-of-plane mechanical properties of the sheet, especially bending and torsional resistance, relative to said layer of cellulose material alone.

The invention particularly aims to provide such a composite cellulose material suitable for use in fields where the paper is highly stressed, especially, but not exclusively for the manufacture of packaging for food, cosmetic or pharmaceutical products, paper for bags, sachets and pouches, a core for honeycomb sandwich panels or multi-ply paper towel or toilet paper structure, or wallpaper, printed paper for promotional use such as papers for posters, inserts or leaflets, papers for domestic use such as paper napkins, paper items such as book covers or an envelope, for example.

To this end, the invention proposes a composite cellulose material comprising:

a layer of cellulose material,

a starch-based reinforcement grid positioned on at least one surface of the cellulose material layer, said reinforcement grid comprising a plurality of meshes delimited by grid wires,

the composite cellulose material having a three-dimensional relief comprising folds at the areas for positioning the reinforcement grid and raised bumps on either side of the plane of the grid delimited by the folds.

According to other aspects, the composite cellulose material according to the invention has the following different characteristics taken alone or according to their technically possible combinations:

the three-dimensional relief has an overall thickness greater than the sum of the thicknesses of the layer of cellulose material and of the reinforcement grid;

the coverage rate of the reinforcement grid on the surface of the cellulose material layer is greater than or equal to 10%, preferably greater than or equal to 20%;

the coverage rate of the reinforcement grid on the surface of the cellulose material layer is less than or equal to 60%, preferably less than or equal to 50%;

the grid wires form square or rectangular meshes;

the grid wires form hexagonal meshes;

the grid wires form honeycomb meshes;

the grid wires form bowtie meshes;

one or more grid wires are sinusoidal;

the reinforcement grid is positioned on the paper layer by screen printing, three-dimensional printing, intaglio, flexography, or spraying via one or more nozzles;

the reinforcement grid is positioned on the surface of the layer of cellulose material according to a quantity comprised between 2 g/m² and 50 g/m² after drying;

the grid wires have a width in the plane of the grid comprised between 0.1 mm and 3 mm, preferably between 0.5 mm and 2.5 mm;

the coverage rate of the reinforcement grid on the surface of the cellulose material layer is comprised between 10% and 60%, preferably between 20% and 50%.

The invention also relates to items made from the composite cellulose material described previously.

Such items can be, especially but not exclusively, a flexible package, such as a food package, for example, a wallpaper, a poster board, preferably of the sandwich type, or even an envelope.

Another subject of the invention concerns a manufacturing method for such a composite cellulose material such as described previously, from a layer of cellulose material. This method is mainly characterized in that it comprises a step consisting of depositing a starch-based reinforcement grid on at least one surface of the cellulose material, said reinforcement grid comprising a plurality of meshes delimited by grid wires, in order to form a three-dimensional relief comprising folds at the areas for positioning the reinforcement grid, and raised bumps on either side of the grid delimited by the folds.

According to other aspects, the manufacturing method according to the invention has the following characteristics taken alone or according to their technically possible combinations:

the reinforcement grid deposited on the cellulose material comprises a starch suspension;

the grid is deposited by screen printing, three-dimensional printing, intaglio, flexography, or spraying via one or more nozzles;

the composition of the reinforcement grid comprises at least one starch suspension having a dry matter content comprised between 5% and 65% by weight when the reinforcement grid is deposited.

DESCRIPTION OF THE FIGURES

Other particular advantages and characteristics of the invention will appear upon reading the description provided as an illustrative and non-limiting example, with reference to the following attached figures:

FIG. 1 is a diagram of a reinforcement grid, wherein the grid wires form honeycomb hexagonal meshes;

FIG. 2 is a diagram of a reinforcement grid, wherein the grid wires form bowtie hexagonal meshes;

FIG. 3 is a diagram of a reinforcement grid, wherein all the grid wires constituting the meshes are sinusoidal;

FIG. 4 is a diagram of a reinforcement grid, wherein the grid wires are orthogonal two by two and form square meshes;

FIG. 5 is a graph illustrating the change in bending resistance of a composite cellulose material according to the invention, comprising a sheet of paper and a reinforcement grid with square meshes in dextrin starch, as a function of the coverage rate of the sheet of paper by the grid;

FIG. 6 is a graph illustrating the change in bending resistance of a composite cellulose material according to the invention, comprising a sheet of paper and a reinforcement grid with square meshes of the mixed dextrin and waxy starch type, as a function of the coverage rate of the sheet of paper by the grid;

FIG. 7 is a graph illustrating the bending resistance of the composite cellulose material for reinforcement grids having different patterns;

FIG. 8 is a graph illustrating the bending resistance of composite cellulose materials for reinforcement grids of different compositions;

FIG. 9 is a graph illustrating the overall thickness of composite materials according to the graph of FIG. 8 ;

FIG. 10 is a graph illustrating the quantity or grammage of composite materials according to the graph of FIG. 8 ;

FIG. 11A is an overhead photograph of a composite material obtained by deposition of a starch reinforcement grid on a layer of Gerstar™ cellulose material;

FIG. 11B is an overhead grazing photograph of the composite material of FIG. 11A;

FIG. 12 is a graph illustrating the bending resistance of composite materials obtained by deposition of a starch reinforcement grid on tracing paper and on blotting paper;

FIG. 13 is a graph illustrating the overall thickness of composite materials according to the graph of FIG. 12 ;

FIG. 14 is a diagram illustrating the determination of the overall thickness.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

The invention concerns a composite cellulose material comprising a layer of cellulose material and a reinforcement grid positioned on at least one surface of the cellulose material layer. The reinforcement grid can be deposited over the entire surface, or over only a part of this surface. Such a composite cellulose material makes it possible to make packages, especially for food products.

It is specified that a “composite material” is a combination of at least two non-miscible components. A synergistic effect is obtained by such a combination, so that a composite material has properties, especially mechanical properties, that each of the components alone do not have, or have to a lesser degree than the composite material. In this case, the first component of the composite cellulose material is the cellulose material layer and the second component is the reinforcement grid.

The cellulose material layer forms the matrix of the composite cellulose material. Such a matrix ensures the cohesion of the structure of the composite cellulose material and transmits the forces exerted on this material to the reinforcement grid.

The reinforcement grid is a composite cellulose material reinforcement and ensures a good mechanical strength thereof.

Depositing the reinforcing layer on the surface of the cellulose material makes it possible to improve the specific out-of-plane mechanical properties of said composite cellulose material layer by increasing its bending resistance, in particular, as well as its torsional resistance.

The layer of cellulose material is preferably a sheet of paper.

Preferably, the sheet of paper has a grammage comprised between 13 g/m² and 140 g/m², preferably between 30 g/m² and 90 g/m². These grammage ranges correspond to a relatively flexible sheet of paper, typically sheets intended for the manufacture of flexible packages such as pouches, bags and sachets, which are particularly preferred for creating the composite cellulose material of the invention.

The reinforcement grid comprises a plurality of meshes delimited by grid wires.

The reinforcement grid comprises starch.

The starch can be native starch or modified starch, for example a dextrin.

Starch is a glucose polymer, generally a mix of amylopectin (branched) and amylose (linear), naturally present in many plants. In the state of the art, two strategies for starch modification are commonly implemented industrially: acidic or enzymatic conversion of the starch in order to generate polymers of lower molecular mass, for example dextrins, and chemical modification of the starch by reaction of the starch hydroxyl groups with functional agents to introduce substitution groups. For example, it is especially a matter of starches such as hydroxypropyl ethers or hydroxypropyl starch.

The native or modified starch can be associated with other constituents in the reinforcement grid. Following the deposition of the starch grid on the surface of the cellulose material layer, the starch shrinks when it dries. Astonishingly, the shrinkage forces exerted during drying of the starch-based formulation previously deposited on the surface of the cellulose layer lead to a deformation gradient in the thickness of the cellulose layer. This deformation is favoured by the wetting of the cellulose layer which occurs between deposition of the grid and the end of drying. A fold is thus formed, that is also called “valley fold” in that a valley is formed in the fold on the side on which the reinforcement grid was deposited, in the places where starch retraction occurs, i.e., at the areas where the grid is deposited. These folds form an angle with the plane of the surface of the cellulose layer that depends, in particular, on the width of the grid lines (or wires) and the quantity deposited. The term “valley fold” is a common term in origami that well describes the present situation.

The formation of these folds at the reinforcement grid forms a relatively homogeneous three-dimensional relief due to the deformation of the cellulose material located between the folds to form raised bumps or “peaks” on either side of the grid plane. This deformation increases the so-called “overall” thickness of the composite material formed, as well as its out-of-plane mechanical properties, especially bending and torsional resistance.

More precisely, “overall thickness” means the distance between the planes tangent to the rough upper and lower surfaces of the cellulose layer, and parallel to each other. The determination of the overall thickness, denoted Ep, is illustrated in FIG. 14 .

The deposition of the starch grid on the surface of the cellulose layer therefore makes it possible to obtain a synergistic effect: a local increase in thickness, at the areas where the grid is deposited, as a result of the local addition of material (starch), as well as an additional effect of increasing the overall thickness, which occurs more globally on the cellulose layer, and more substantially at the areas where the grid is deposited, according to the deformation gradient in said thickness of said cellulose layer. In other words, the thickness of the composite material obtained is greater than the sum of the thicknesses of the layer of cellulose material and of the reinforcement grid.

This increase in the overall thickness is reflected by an improvement in the out-of-plane mechanical properties of the composite material, especially its bending and torsional resistance, greater than what was initially expected by the applicant, and which would be obtained by just increasing the local thickness due to the addition of material.

The overall thickness is measured according to standard ISO12625-3 as being the distance between a fixed reference plate on which the sample rests and a parallel probe which exerts a specified load on the surface under test.

A precision counterbalanced micrometer is used which comprises two parallel and flat horizontal faces, between which a specimen of the material of interest is placed. The upper circular probe has a diameter greater than (35.7±0.1) mm, or a nominal surface area of 10.0 cm². The pressure between the two faces of the micrometer is (2.0±0.1) kPa.

The starch grid offers the additional advantage of being less expensive. The starch can be present in the form of a starch suspension, which simplifies the deposition of the grid, the starch suspension being actually relatively easy to implement and being appropriately distributed over the surface of the cellulose material. A certain number of products can be added to the starch suspension, such as, especially, rheology modifiers, pigments, plasticizers or surfactants in order to modulate or improve the properties when the grid is created or when it is used.

Materials other than starch can be envisaged without exceeding the scope of the invention. However, these materials must have a similar shrinkage to starch in order to be able to lead to an increase in overall thickness. Shrinkage means the dimensional variations and associated phenomena observed during desiccation [3].

The coverage rate of the reinforcement grid on the surface of the cellulose material layer is greater than or equal to 10%, preferably greater than or equal to 20%.

The coverage rate of the reinforcement grid can be defined as being the ratio between the portion of the surface of the cellulose material layer which is covered by the reinforcement grid and the portion of said surface which is free, i.e., not covered by said reinforcement grid.

With a coverage rate less than 10%, the quantity of reinforcement grid deposited is low and more difficult to measure and repeat. Moreover, the improvement of the out-of-plane mechanical properties, especially bending and torsional resistance is reduced.

The coverage rate of the reinforcement grid on the surface of the cellulose material layer is less than or equal to 60%, preferably less than or equal to 50%.

A coverage rate greater than 60% does not further improve the out-of-plane mechanical properties, while significantly rigidifying the composite material formed, especially in-plane, which is not necessarily desirable depending on the final application of the composite material. A coverage rate less than 60% but greater than 50% only very slightly improves the out-of-plane mechanical properties and rigidifies the composite material in plane.

According to one embodiment, the coverage rate is comprised between 10% and 60%, preferably between 10% and 50%, and more preferably comprised between 20% and 50%. This coverage rate permits a good compromise between improving the overall thickness and bending and torsional resistance and the manufacturing cost of the composite cellulose material by using a moderate quantity of reinforcement grid composition.

With a coverage rate greater than 90%, the surface of the areas not covered by the reinforcement grid would be insufficient to be able to form the bumps in the cellulose material.

The reinforcement grid is positioned on the paper layer by screen printing, three-dimensional printing, intaglio, flexography, or spraying via one or more nozzles.

Screen printing is a printing technique using as a printing form a cloth, also called a screen, the meshes of which are sealed on the areas which should not be printed. The ink is deposited on the back of the cloth and a doctor blade system forces the ink through the unsealed meshes of the cloth to come into contact with the material to be printed. This technique is advantageous for depositing the reinforcement grid since it uses inks of medium viscosities (viscosities between 500 and 5000 mPa·s) and allows performing localized deposits ranging from 5 to 120 μm of ink thickness before drying.

According to the intaglio technique, recesses are engraved in the printing form (usually a metal plate). The plate is covered with ink and then scraped to leave ink only in the recesses of the printing form. This printing form is pressed onto the paper to transfer the ink from the recesses of the printing form to the surface of the sheet. This process is especially advantageous to deposit ink thicknesses from 10 to 60 μm before drying but requires a particularly viscous ink (viscosity between 10,000 and 25,000 mPa·s) and exerts pressure at transfer reducing the thickness of the cellulose layer.

Flexography allows printing the reinforcement grid by using a flexible printing form in relief.

The reinforcement grid preferably represents an increase of grammage after drying comprised between 2 and 50 g/m², preferably between 5 and 20 g/m². In other words, this corresponds to the quantity of the reinforcement grid which is positioned on the surface of the cellulose material layer.

The grid wires preferably have a width in the plane of the grid comprised between 0.1 mm and 3 mm, preferably between 0.5 mm and 2.5 mm;

The reinforcement grid meshes can have various patterns. For example, it can be a square, rectangular, hexagonal, honeycomb, bowtie or sinusoidal pattern.

Some examples of patterns are shown in FIGS. 1, 2 and 3 . For these various examples, the meshes are dimensioned to have a coverage rate of approximately 30%.

In reference to FIG. 1 , the reinforcement grid comprises hexagonal-patterned meshes. The hexagon comprises six parallel sides two by two and of the same length, denoted b, equal to 5 mm. The thickness of the wires forming the mesh, denoted e, is equal to 1 mm.

In reference to FIG. 2 , the reinforcement grid comprises bowtie or hourglass-patterned meshes.

According to a first example, the length L of the lateral side of the bowtie is equal to 6 mm, the length H of the base is equal to 12 mm, the angle θ between the base and the lateral side is equal to 45°, and the thickness e of the wires forming the mesh is equal to 0.8 mm.

According to a second example, the length L of the lateral side of the bowtie is equal to 5 mm, the length H of the base is equal to 10 mm, the angle θ between the base and the lateral side is equal to 60°, and the thickness e of the wires forming the mesh is equal to 0.8 mm.

In reference to FIG. 3 , the reinforcement grid comprises sinusoidal-patterned meshes. Each mesh is formed by two warp wires which extend opposite each other in a substantially vertical direction, the curvature of one wire being reversed with respect to that of the other wire, and by two weft wires which extend facing each other in a substantially horizontal direction, the curvature of one wire being reversed with respect to that of the other wire.

According to a first example, the curve deviation, denoted a, of each wire with respect to an equivalent straight wire is equal to 1.5 mm, the space I between two opposite sides of the equivalent square is equal to 5 mm, and the thickness e of the wires forming the mesh is equal to 0.8 mm.

According to a second example, the curve deviation, denoted a, of each wire with respect to an equivalent straight wire is equal to 1.8 mm, the space I between two opposite sides of the equivalent square is equal to 6 mm, and the thickness e of the wires forming the mesh is equal to 1 mm.

The invention also relates to a method for manufacturing a composite cellulose material such as previously described.

The method comprises a step consisting of depositing the reinforcement grid on at least one surface of the cellulose material

According to a preferred embodiment, the grid is deposited by screen printing, three-dimensional printing, intaglio, flexography, or spraying via one or more nozzles.

Preferably, the composition of the reinforcement grid comprises at least one starch suspension having a dry matter content comprised between 5% and 65% by weight when the reinforcement grid is deposited.

EXAMPLES

Example 1: determining the bending resistance of composite cellulose materials comprising reinforcement grids with different coverage rates

A rectangular mesh reinforcement grid was printed by screen printing on a paper of 55 g/m² dry grammage, to obtain the corresponding composite cellulose material. The grid is shown in FIG. 4 .

The grid thickness is 20 μm for a line width, denoted a, comprised between 0.25 mm and 2.5 mm according to the percentage of coverage between 20% and 75%. The medians of the lines are spaced from one another by 5 mm. The grid has a dimension of 300 mm by 200 mm.

Two formulations were tested for the reinforcement grid:

a dextrin,

a mix based on 90% dextrin and 10% waxy starch (rich in amylopectin).

The composite cellulose material obtained was dried in a furnace at a temperature of 60° C. for 1 minute and 30 seconds.

The results obtained with the dextrin-based reinforcement grid of low molecular weight are illustrated in FIG. 5 . The graph of FIG. 5 shows the bending resistance (mNm) at 15° according to the coverage rate (%). The photographs illustrate the appearance of the surface of the composite material obtained.

The bending resistance, measured according to the standard ISO 2493-1:2010, goes from 0.07 mNm (null coverage rate, no reinforcement grid) in the machine direction, denoted MD, to 0.22 mNm for the composite cellulose material having a coverage rate of 50%.

In the cross machine direction, the bending resistance goes from 0.04 mNm to 0.12 mNm.

The results obtained with the reinforcement grid based on the mixture of 90% dextrin and 10% waxy starch are illustrated in FIG. 6 and are similar to those of FIG. 5 . The graph of FIG. 6 shows the bending resistance (mNm) at 15° according to the coverage rate (%).

The bending resistance, measured according to the standard ISO 2493-1:2010, goes from 0.07 mNm (null coverage rate, no reinforcement grid) in the machine direction MD, to 0.21 mNm for the composite cellulose material having a coverage rate of 50%.

In the cross machine direction, the bending resistance goes from 0.04 mNm to 0.13 mNm.

Example 2: determining the bending resistance of composite cellulose materials comprising reinforcement grids with different patterns

Eight composite cellulose materials comprising grids fabricated from the same composition and having different patterns were tested in order to determine their bending resistance. They are compared to the material (a) not comprising the grid, which is a flexible packaging paper sold by the Ahlstrom-Munksjö company. The coverage rate is approximately 30% for the eight materials, except material (c).

The composite cellulose materials are the following:

material (a): Gerstar™ material with no grid,

material (b): rectangular grid,

material (c): rectangular grid whose coverage rate is 51%,

material (d): hexagonal grid of the honeycomb type,

material (e): hexagonal grid of the bowtie type; the angle θ between the base and the lateral side is equal to 45°,

material (f): hexagonal grid of the bowtie type; the angle θ between the base and the lateral side is equal to 60° and the wire thickness wire is 1 mm,

material (g): hexagonal grid of the bowtie type, the angle θ between the base and the lateral side is equal to 60° and the wire thickness is 0.8 mm,

material (h): sinusoidal grid, the wire thickness e is 1 mm,

material (i): sinusoidal grid, the wire thickness e is 0.8 mm,

The results are shown in the form of a graph in FIG. 7 . The graph of FIG. 7 shows the bending resistance (mNm) as a function of the different materials.

The measurements are performed in the machine direction MD as well as the cross machine direction CD. Photographs illustrate the appearance of the surface of the different composite materials.

According to the graph, it is observed that the improvement in bending resistance of the composite cellulose material is generally greater with materials with the honeycomb type grid (d), bowtie type hexagonal grid (f) of 1 mm thickness and sinusoidal grid (h) of 1 mm thickness compared to materials with orthogonal grid.

Example 3: determining the bending resistance of composite cellulose materials comprising reinforcement grids of different materials

Eight composite cellulose materials comprising grids fabricated from several different compositions and having meshes with a pattern identical to hexagonal sinusoidal meshes, were tested in order to determine their bending resistance. The coverage rate is approximately 30% for the eight materials. The eight composite materials comprise the same cellulose material: Gerstar™ flexible packaging material.

The different grid compositions are the following:

material (a): Gerstar™ material with no grid,

material (b): polyvinyl alcohol (PVOH) grid,

material (c): water grid,

material (d): hydroxypropyl starch grid,

material (e): grid in mixture of 90% dextrin and 10% waxy starch,

material (f): grid in mixture of 85% dextrin and 15% waxy starch,

material (g): grid in mixture of 80% dextrin and 20% waxy starch,

material (d): dextrin grid,

The results are shown in the form of a graph in FIG. 8 . The graph of FIG. 8 shows the bending resistance (mNm) as a function of the different materials.

The overall thickness (mm) of the different cellulose materials is shown in FIG. 9 , and the grammage (g/m²) of the different cellulose materials is shown in FIG. 10 .

The measurements are performed in the machine direction MD as well as the cross machine direction CD.

According to the graph of FIG. 8 , it is observed that the five starch grids achieve a very substantial gain in bending resistance relative to the reference. This is explained by the synergistic effect obtained by both a local increase in thickness of the material at the areas where the grid is deposited due to the local addition of starch and an additional effect of increasing the overall thickness.

A photograph of the composite material obtained by depositing the starch grid is shown in FIG. 11A. A grazing view photograph of this same material is shown in FIG. 11B.

The PVOH and water grids give less good results than the starches: the gain in bending resistance relative to the reference is relatively low. This is explained by the fact that the synergy observed for the starch grids does not occur. There is only a local increase in thickness of the material at the areas where the grid is deposited due to the deposition of material.

A starch grid was also deposited on other cellulose materials: tracing paper and blotting paper. The results are shown in the graphs of FIGS. 12 and 13 . The graph of FIG. 12 shows the bending resistance (mNm) as a function of the different materials. The graph of FIG. 13 shows the overall thickness (mm) as a function of the different materials. In these figures, (a) Clq and (e) Clq correspond to the uncovered tracing paper (a) and covered with the grid of the 90% dextrin and 10% waxy starch mixture (e), respectively, and (a) Bvd and (e) Bvd correspond to the blotting paper not covered with the grid (a) and covered with the grid of the 90% dextrin and 10% waxy starch mixture (e), respectively.

It is observed that the gain in bending resistance is similar to that obtained with the flexible packaging paper Gerstar™ of the graph of FIG. 8 . This effect is surprising for the blotting paper insofar as, given the absorbent properties of this material, the starch suspension partially penetrates into the thickness of the sheet. We might therefore have expected that the folds observed with Gerstar™ paper (which is not absorbent) would not form on blotting paper.

REFERENCES

[1]: D. Wang M. M. Abdallan & W. Zhang. Buckling optimization design of curved stiffeners for grid-stiffened composite structures. Composite Structures, vol. 159, p. 656-666, 2017.

[2]: A. Rafsanjani, D. Pasini. Bistable auxetic mechanical metamaterials inspired by ancient geometric motifs. Extreme Mechanics Letters, vol. 9 p. 291-296, 2016.

[3]: G. M. Laudone, G. P. Matthews and P. A. C. Gane, “Coating Shrinkage during Evaporation: Observation, Measurement and Modelling within a Network Structure,” Tappi 8th Advanced Coating Fundamentals Symposium, Chicago, 8-10 May 2003, pp. 116-129. 

1. A composite cellulose material comprising: a layer of cellulose material, a starch-based reinforcement grid positioned on at least one surface of the cellulose material layer, said reinforcement grid comprising a plurality of meshes delimited by grid wires, the composite cellulose material having a three-dimensional relief comprising folds at the areas for positioning the reinforcement grid and raised bumps on either side of the plane of the grid delimited by the folds.
 2. The composite cellulose material according to claim 1 wherein the three-dimensional relief has an overall thickness greater than the sum of the thicknesses of the layer of cellulose material and of the reinforcement grid.
 3. The composite cellulose material according to claim 1, wherein a coverage rate of the reinforcement grid on the at least one surface of the cellulose material layer is greater than or equal to 10%, preferably greater than or equal to 20%.
 4. The composite cellulose material according to claim 1, wherein a coverage rate of the reinforcement grid on the at least one surface of the cellulose material layer is less than or equal to 60%, preferably less than or equal to 50%.
 5. The composite cellulose material according to claim 1, wherein the grid wires form square or rectangular meshes.
 6. The composite cellulose material according to claim 1, wherein the grid wires form hexagonal meshes.
 7. The composite cellulose material according to claim 6, wherein the grid wires form honeycomb type meshes.
 8. The composite cellulose material according to claim 6, wherein the grid wires form bowtie type meshes.
 9. The composite cellulose material according to claim 1, wherein one or more grid wires are sinusoidal.
 10. The composite cellulose material according to claim 1, wherein the reinforcing grid is positioned on the cellulose material layer by screen printing, three-dimensional printing, intaglio, flexography, or spraying via one or more nozzles.
 11. The composite cellulose material according to claim 1, wherein the reinforcement grid is positioned on the at least one surface of the layer of cellulose material according to a quantity comprised between 2 g/m² and 50 g/m² after drying.
 12. The composite cellulose material according to claim 1, wherein the grid wires have a width in a plane of the grid comprised between 0.1 mm and 3 mm, preferably between 0.5 mm and 2.5 mm.
 13. The composite cellulose material according to claim 1, wherein a coverage rate of the reinforcement grid on the at least one surface of the cellulose material layer is comprised between 10% and 60%, preferably between 20% to 50%.
 14. A flexible package comprising a composite cellulose material according to claim
 1. 15. A wallpaper comprising a composite cellulose material according to claim
 1. 16. A notice board, preferably sandwich type, comprising a composite cellulose material according to claim
 1. 17. An envelope comprising a composite cellulose material according to claim
 1. 18. A method for manufacturing a composite cellulose material according to claim 1 from a layer of cellulose material, comprising: depositing a starch-based reinforcement grid on at least one surface of the cellulose material, said reinforcement grid comprising a plurality of meshes delimited by grid wires, in order to form a three-dimensional relief comprising folds at the areas for positioning the reinforcement grid, and raised bumps on either side of the grid delimited by the folds.
 19. The manufacturing method according to claim 18, wherein the reinforcement grid deposited on the cellulose material comprises a starch suspension.
 20. The manufacturing method according to claim 18, wherein the grid is deposited by screen printing, three-dimensional printing, intaglio, flexography, or spraying via one or more nozzles.
 21. The manufacturing method according to claim 18, wherein a composition of the reinforcement grid comprises at least one starch suspension having a dry matter content comprised between 5% and 65% by weight when the reinforcement grid is deposited. 